Simplify building host tools during DIRDEPS_BUILD

The whole point of the DIRDEPS_BUILD is to avoid tree walks
and basically build everything in a single pass.
We use the pseudo MACHINE "host" to represent the build host.

When the build host is not FreeBSD or is an older version of FreeBSD
it may need some help to build host-tools.

The directory tools/build does this - building libegacy.

local.sys.mk: create a pseudo option MK_host_egacy to indicate
if tools/build needs to be built for "host".

local.dirdeps.mk: set MK_host_egacy.host to ${MK_host_egacy}
all other DEP_MACHINES will get "no"

This allows a Makefile.depend.options in makefs etc to cause tools/build
to be built for host but only if necessary.

local.init.mk: use ISYSTEM as arg to -isystem so that it can be overridden.
The default remains ${STAGE_INCLUDEDIR}

src.init.mk: if MACHINE is host and we are not FreeBSD
set some MK_ flags the same as tools/build/mk/Makefile.boot.pre and
include src.init.${.MAKE.OS:tl}.mk if it exists.

For older versions of FreeBSD add libegacy when building PROGs for "host"

Also instead of -isystem${STAGE_INCLUDEDIR} we want
-I${STAGE_INCLUDEDIR} and -isystem/usr/include so we override ISYSTEM.
This means any headers we stage for "host" will take precedence over
system headers but #include_next will DTRT.

src.init.linux.mk: add
-I${SRCTOP}/tools/build/cross-build/include/linux
and generally deal with building host tools on Linux.
Eg. static linking does not work so set NO_SHARED= no
Override some HAVE_ flags.

src.sys.env.mk: on linux awk throws an warning about # in newvers.sh
just send stderr to /dev/null

Reviewed by: jrtc27, arichardson
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39744